The invention relates to a method for converting an analog picture signal, corresponding to consecutive fields, into an amplitude discrete output signal, comprising comparing the picture signal with a plurality of equispaced reference values having value intervals between them, producing, at an output, an amplitude-discrete intermediate signal which denotes the value interval in which the value of the picture signal is located, periodically changing the positions of the picture signal, on the one hand, and the reference values, on the other hand, relative to each other by a fraction of a value interval, and, on a picture display device, generating an optical value which corresponds to the average value of the amplitude-discrete output signal formed from the average value of the intermediate signal values of mutually corresponding picture elements.
The German Offenlegungsschrift 30 15 141, corresponding to U.S. Pat. No. 4,352,123, discloses a color television receiver comprising an analog-to-digital converter, to whose analog input a composite color signal is applied and which, from the amplitude thereof, forms a parallel binary word as an output signal. The output signal is inter alia applied to a shift register arrangement producing a delay equal to the line period of the color television picture. In addition, the color television receiver comprises a binary computer stage which forms the arithmetic mean from the input and output signals of the shift register arrangement. Because of the high frequency values to be processed, the analog-to-digital converter is in the form of a parallel-analog-to-digital converter. To reduce the cost of such an analog-to-digital converter by 50%, without affecting the digital resolution, either the reference voltage applied to the analog-to-digital converter or the input signal are shifted, for the duration of each second line, by the voltage corresponding to the value of half-a-bit of the parallel binary word, when the parallel binary word of the highest value which is capable of being displayed, corresponds to the reference voltage.
In principle, the prior art arrangement is obviously based on the recognition that the digital resolution of an analog-to-digital converter, in which an analog signal, for example an analog picture signal, is compared with a plurality of equispaced reference values, can be enhanced because of the fact that the analog signal is compared with the reference values in several consecutive (comparison) steps, that between each of these two steps either the reference value or the analog signal are shifted through a portion of the value interval between two reference values without changing their relative distances, and that the amplitude-discrete intermediate signals comprised in the relevant steps are averaged. For example, a value of the analog signal can be compared in two steps with the reference values, the analog signal and the reference values being shifted relative to each other through half-a-value interval between the two steps. If then the average value of the amplitude-discrete intermediate signal values obtained in the two steps is formed, this will provide an indication whether the value of the analog signal is located in the lower or in the upper half of the relevant value interval. When the reference values, or the value intervals, are sequentially numbered with a binary number, halving the value interval as described above is equal to extending the binary number by one position, or equal to doubling the resolution of the analog-to-digital converter. Similarly, a finer sub-division of the value intervals is alternatively possible, namely when the number of steps effected to obtain a value of the analog signal is increased and the shift between the analog signal, on the one hand, and the reference values, on the other hand, is made finer accordingly. Generally, for one value of the analog signal, an (second) integral number of comparison steps are effected; between each two comparison steps, the analog and the reference values are shifted relative to each other over a value which corresponds to a fraction of a value interval, more specifically, a value interval multiplied by the reciprocal of this (second) integer.
In principle, acting thus, an optionally fine subdivision of the value intervals of the analog-to-digital converter is possible. Using such an analog-to-digital converter of a simple construction whose reference values are spaced apart by comparatively large value intervals, a very fine resolution can ultimately be obtained in this way. This procedure has the disadvantage, in addition to the increasing overall duration of digitizing an analog signal because of the increasing number of comparison steps, that the results of all the comparison steps must be stored intermediately and be linked together by an external computer. Moreover, to accelerate the total procedure, a comparatively high switching frequency is required for shifting the reference values or the analog signal, respectively, which may cause transients and consequently invalidate the result of the analog-to-digital conversion.
The circuit arrangement disclosed in the German Offenlegungsschrift No. 30 15 141 (U.S. Pat. No. 4,352,123) has the disadvantage that the reference voltage, or the input signal, are shifted with a comparatively high switching frequency. As analog signals are processed and a very accurate setting of the desired voltages is necessary, very expensive circuit arrangements are required therefor, for example, to suppress overshoot.
The German Offenlegungsschrift No. 28 43 706, corresponding to U.K. Patent Specification No. 2,006,569, discloses a comparison stage of the type set forth in the opening paragraph in which the position of the reference values for the odd fields is changed by half the distance between two adjacent reference values relative to the position of the reference values for the even fields. This results in the switching frequency for shifting the reference voltage relative to the input signal being reduced to the field frequency.
A further disadvantage of the circuit arrangement described in the German Offenlegungsschrift No. 30 15 141 (U.S. Pat. No. 4,352,123) is that errors in the analog-to-digital converter become manifest in a very distracting manner. Thus, it may happen that a reference value occupies an incorrect position. Particularly when an area having a slightly varying luminance or color is displayed on the picture display device, such an error produces continuous, perpendicular, sudden changes in the color or the luminance.